Fluid–structure interactions of skeleton-reinforced fins:performance analysis of a paired fin in lift-based propulsion

SUMMARY We investigate the thrust generation capacity of a thin foil consisting of a membrane strengthened by embedded rays that is geometrically, structurally and kinematically similar to pectoral fins of bony fishes during lift-based labriform locomotion. Our numerical model includes a fully nonlinear Euler–Bernoulli beam model of the skeleton and a boundary-element model of the surrounding flow field. The fin undergoes a dorso–ventral flapping activated by rotations of the rays. Both the trailing edge vortices(TEV) and the leading edge vortices (LEV) are accounted for and modeled as shear layers. The thrust generation and propulsion efficiency are examined and documented. Our results show that synchronization of rays is pivotal to the performance of the system. A primary factor that determines the performance of the fin is phase lags between the rays, which create variations of the effective angle of attack at the leading edge as well as shape changes throughout the fin surface. Structural flexibility of the rays leads to passive deformations of the fin, which can increase the thrust generation and the propulsion efficiency.